Note: Descriptions are shown in the official language in which they were submitted.
1
ANODE FOR CATHODIC PROTECTION AND METHOD FOR MANUFACTURING THE
SAME
FIELD OF THE INVENTION
The invention relates to the field of cathodic protection of reinforced
concrete structures, and
in particular to a design of anode particularly efficient in terms of
electrical resistance per unit
length and of flexibility, and particularly safe to install and handle.
The invention also relates to the method of production of such anode.
BACKGROUND OF THE INVENTION
Corrosion phenomena affecting reinforced concrete structures are well known in
the art. The
steel reinforcement inserted in the cenrientitious structures to improve the
mechanical
properties thereof normally works in a passivation regime induced by the
alkaline concrete
environment; however, after some time, the ion migration across the porous
surface of the
concrete causes a localised attack to the protective passivation film.
Particularly worrying is
the attack by chlorides, which are virtually present in all kinds of
environments where the
reinforced concrete structures are employed, and to an even higher extent
where an exposure
to brackish water (bridges, pillars, buildings located in marine zones),
antifreeze salts (bridges
and road structures in cold climate zones) or even seawater, such as for
instance in the case
of piers and docks, takes place. The critical value of chloride exposure has
been esteemed
around 0.6 kg per cubic metre of concrete, beyond which the passivation state
of the
reinforcing steel is not guaranteed. Another form of concrete decay is
represented by the
carbonatation phenomenon, that is the formation of calcium carbonate by
reaction of the lime
of the cementitious mixture with atmospheric carbon dioxide. Calcium carbonate
lowers the
concrete alkali content (from pH 13.5 to pH 9) bringing iron in an unprotected
state. The
presence of chlorides and the simultaneous carbonatation represents the worst
of conditions
for the preservation of the reinforcing bar of the structures. The corrosion
products of steel are
more voluminous than steel itself, and the mechanical stress resulting from
their formation
may lead to concrete delamination and fracturing phenomena, which translate
into huge
damages from the point of view of economics besides the one of safety. For
this reason, it is
known in the art that the most effective method for indefinitely prolonging
the lifetime of
reinforced concrete structures exposed to the atmospheric agents, even in the
case of relevant
salt concentrations, consists of cathodically polarising the steel
reinforcement. In this way, the
latter becomes the site of an oxygen cathodic reduction, suppressing the
anodic corrosion and
dissolution reactions. Such system, known as cathodic protection of reinforced
concrete, is
practised by coupling anodic structures of various kinds to the concrete,
respect to which the
reinforcement to be protected
CA 2808397 2018-04-06
CA 02808397 2013-02-14
WO 2012/035107 PCT/EP2011/066021
2
acts as a cathodic counterelectrode; the electrical currents involved
supported by an external
rectifier transit across the electrolyte consisting of the porous concrete
partially soaked with a
salty solution.
The anodes commonly used for the cathodic protection of reinforced concrete
consist of a
titanium substrate coated with transition metal oxides or other types of
catalysts for anodic
oxygen evolution. As the substrate it is possible to make use of other valve
metals, either pure
or alloyed; pure titanium is however the largely preferred choice for the sake
of cost.
European Patent EP458951 discloses a grid-type electrodic structure for
cathodic protection
consisting of a plurality of metal ribbons having an electrocatalytic coating,
said metal ribbons
having voids of different geometries.
This type of ribbons can be manufactured by punching of solid metal ribbons or
more commonly
by the traditional methods of metal expansion wherein a metal sheet is
expanded by pressuring
and punching through a series of knives arranged orthogonal to the advancement
direction of
the ribbon itself. This first step allows obtaining an expanded metal sheet.
Such sheet is then
subjected to a second step of cutting suitable for obtaining ribbons of the
required dimensions.
Said expanded metal ribbons present meshes having voids of rhomboidal shape
with the major
diagonal oriented orthogonal to the ribbon length.
This method of manufacturing has the inconvenience of producing metal ribbons
with meshes
having cutting side protrusions automatically formed during the operation of
cutting, making
these anodes difficult to handle and the installation phase accordingly
dangerous.
Metal ribbons with smooth lateral edges are disclosed in Canadian Patent
Application CA
2078616 Al; by the method described this document, the ribbons obtained
present a
continuous longitudinally-extending solid section of a certain width, which is
invariably formed in
the manufacturing process and which can only be used for spot-welding. In
present-day
cathodic protection systems, however, it is preferred not to weld ribbon
anodes at all, but rather
to overlay them directly to the reinforcement with plastic spacers arranged in-
between. In such
case, the longitudinally-extending solid section is just a loss of material,
especially because this
solid section invariably gets coated with precious metals during the
application of the catalytic
layer. Such catalytic layer however cannot work properly on a non-foraminous
structure and
affects the calculation of the actual current density impressed to the anodic
structure, thereby
complicating the design of the overall cathodic protection system.
3
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a
method of
manufacturing an anode comprising the following steps: running of a metal
ribbon through an
.. expanding device equipped with at least one row of knifes of a first
predetermined length
arranged parallel to the direction of the ribbon displacement, expansion of
the metal ribbon by
means of pressing and punching action of said at least one row of knives,
formation of lateral
edge profiles of the expanded metal ribbon by means of pressing and punching
action of a
last row of knives having blades of a second predetermined length higher than
said first length.
Under one aspect, the invention relates to an anode in form of mesh ribbon for
systems of
cathodic protection, for instance of cathodic protection of reinforced
concrete structures,
overcoming the inconveniences of the prior art, whose edges are substantially
free of
discontinuities in form of cutting protrusions and have a sinusoidal shape.
In the context of the present description reference is made, for the sake of
simplicity, to
cathodic protection of reinforced concrete structures; it is understood that
the invention may
be practised in the field of cathodic protection in general, for instance
comprising the cathodic
protection of metal tank bottoms.
Under another aspect, the invention relates to a method for manufacturing said
anode.
Under a further aspect, the invention relates to a cathodic protection system
comprising at
least one anode in form of mesh ribbon whose edges are substantially free of
cutting
protrusions.
Some of the most significant results obtained by the inventors are presented
in the following
description, which is merely provided by way of example without wishing to
limit the invention.
.. The anode according to the invention consists of a ribbon of expanded metal
characterised
by meshes with rhomboidal shaped voids with the major diagonal oriented along
the direction
of the ribbon length. In one embodiment, the lateral edges of the ribbon have
a sinusoidal
profile and are free of cutting protrusions.
The inventors have surprisingly noticed that an anode for cathodic protection
as hereinbefore
described displays a remarkably reduced ohmic resistance per unit length, for
instance up to
4-fold reduced, with respect to the anodes of the prior art.
CA 2808397 2018-04-06
3a
The lower electrical resistance makes possible to reduce the number of
electrical connections,
for instance in a grid system, with sensible savings of material and
installation time.
In one embodiment, the metal mesh ribbon is made of titanium.
CA 2808397 2018-04-06
CA 02808397 2013-02-14
WO 2012/035107 PCT/EP2011/066021
4
In another embodiment, the metal mesh ribbon is coated with a catalytic
coating containing
noble metals or oxides thereof.
In one embodiment, the dimensions of the ribbon can have a width ranging from
3 mm to 100
mm with a thickness of 0.25 mm to 2.5 mm and a length of 1 m to 150 m.
BRIEF DESCRIPTION OF THE DRAWINGS
For the sake of a better understanding of the invention, reference will be
made to the following
drawings, having the purpose of depicting some preferred embodiments thereof
without limiting
its extent.
- Fig. 1A shows a top-view of a traditional expanded metal anode.
- Fig. 1B shows a top-view of an expanded metal anode according to the
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
In detail, Fig. 1A shows a top view of the traditional anode in which are
distinguishable cutting
protrusions 1 due to the manufacturing method including a cutting step, the
rhomboidal
geometry with major diagonal 3 of rhomboidal voids arranged in the direction
of the ribbon width
.. and the minor diagonal 4 of the same arranged in the direction of the
ribbon length.
Fig. 1B shows a top-view of the anode according to the invention in which are
distinguishable
non-cutting blunt lateral edges 2, the rhomboidal geometry with major diagonal
3 of rhomboidal
voids arranged in the direction of the ribbon length and the minor diagonal 4
of the same
arranged in the direction of the ribbon width.
EXAMPLE
Some of the most significant results obtained by the inventors are reported in
Table 1, wherein
.. ohmic resistance data of representative anodes of the invention are
compared to traditional
anodes. Anodes labelled A and B are anodes of rhomboidal geometry with the
major diagonal
of rhomboids oriented orthogonal to the ribbon length likewise depicted in
Fig. 1A, traditionally
obtained by longitudinal expansion with respect to the displacement direction
of a solid metal
ribbon. Anodes labelled C and D are anodes of rhomboidal geometry according to
one
embodiment of the invention, likewise depicted in Fig. 1B.
CA 02808397 2013-02-14
WO 2012/035107 PCT/EP2011/066021
Anodes C and D were prepared by orthogonal expansion with respect to the
displacement
direction of a solid metal ribbon allowed to run in an apparatus along a
parallel row of knives
which expand the solid ribbon in an orthogonal direction by pressuring and
punching. The
ribbon manufacturing is completed by means of a last series of knives, having
blades of
5 predefined length higher than the blades of previous knives, which upon
applying a pressure
are suitable for modelling the lateral edge of the ribbon as depicted in Fig.
1B. Besides the
advantages already explained in terms of conductivity due to the anode
geometry, this method
has the advantage of providing an expanded metal ribbon free of longitudinally-
extending solid
sections which, being not subsequently cut, does not present any cutting edge
and is therefore
much safer and easy to handle during the installation. This method moreover
allows to
advantageously obtain a metal ribbon of the desired length directly upon
completion of the
expansion. Such method of production furthermore allows obtaining ribbons of
higher length
than the traditional method thereby facilitating big size installation which
would require
connections of multiple ribbons, with a lower solidity of the overall anodic
system.
From the data reported in the table it can be noticed that for a given width,
the anodes of the
invention display an ohmic resistance about 60% lower.
Table 1
Anodes in accordance with Fig. 1A R- Ohmic Resistance
A - 20mm wide 0.22 Ohm/m
B - 10mm wide 0.43 Ohm/m
Anodes in accordance with Fig. 1B R- Ohmic Resistance
C - 20mm wide 0.088 Ohm/m
D - lOmm wide 0.177 Ohm/m
The previous description is not intended to limit the invention, which may be
used according to
different embodiments without departing from the scopes thereof, and whose
extent is
univocally defined by the appended claims.
Throughout the description and claims of the present application, the term
"comprise" and
variations thereof such as "comprising" and "comprises" are not intended to
exclude the
presence of other elements or additives.
CA 02808397 2013-02-14
WO 2012/035107 PCT/EP2011/066021
6
The discussion of documents, acts, materials, devices, articles and the like
is included in this
specification solely for the purpose of providing a context for the present
invention. It is not
suggested or represented that any or all of these matters formed part of the
prior art base or
were common general knowledge in the field relevant to the present invention
before the priority
date of each claim of this application.
